Magnetic fields create eddy currents within certain types of materials in objects in their path. The eddy currents in turn affect the magnetic field as observed from outside the objects. Cracks, discontinuities, holes, and changes in the material content all affect the eddy-current flow within an object and also affect the magnetic field external to the object. Accordingly, magnetic fields can be used to scan materials to determine if the materials contain inconsistencies and anomalies (such as cracks or corrosion) that affect the magnetic field.
Remote-field eddy-current techniques can be used to scan materials. Remote-field eddy-current techniques (RFEC) generally involve detecting magnetic-field changes caused by anomalies on a surface of, and/or hidden in, a structure due to the RFEC technology's double-wall-transmission feature.
In contrast, conventional eddy-current (EC) techniques generally involve detecting magnetic-field changes caused by anomalies on surface and near-surface areas. This is because the direct coupling is dominating in conventional eddy-current technique. There are not separate drive and pickup coils/units used in EC absolute and differential modes. In the reflection mode, in spite of the geometrical separation of the two coils/units, the direct coupling of excitation unit(s) and sensor unit(s) is still dominating. Generally, the magnetic separation of drive and pickup units, using shields and magnetic circuits, for an RFEC probe is greater than that of an EC probe working in reflection mode. Changes to an observed RFEC or conventional EC signal can be caused by undesirable anomalies, even those hidden in a surface, such as cracks, voids, internal or surface corrosion, embedded foreign objects, alloy-composition changes, etc., as well as by expected inherent features of the object being examined, such as joints and fasteners.
U.S. Pat. No. 6,636,037 issued to Tianhe OUYANG on Oct. 21, 2003, is titled “Super Sensitive Eddy-Current Electromagnetic Probe” and is incorporated herein by reference. This patent describes devices and methods for improved inspections of conducting structures of different shapes. An eddy-current probe includes an excitation coil unit, a magnetic detector within the probe, a signal-conditioning/preamplifier circuit within the probe, and a signal channel. The excitation coil unit is shielded on substantially all sides except an emission face that transmits an alternating magnetic signal to a conducting (e.g., metal) object, such that the metal object modifies the alternating magnetic signal. The magnetic detector within the probe is also shielded on substantially all sides except a reception face, such that the alternating magnetic signal as modified by the metal object is received into the shielded magnetic detector and converted into a first electrical signal. The signal-conditioning/preamplifier circuit within the probe is shielded on substantially all sides, and provided with electrical power. The shielded preamplifier provides detection for very small signals, such as from magnetic probing of aircraft-skin metals. Other embodiments include a traveling-wave excitation structure and a multiple-phase driving circuit, some of which include the shielded pre-amplifier, and others of which are not shielded. An eddy scope is described that provides a multiple-phase excitation signal to various different probes.
U.S. patent application Ser. No. 11/114,507 (now U.S. Pat. No. 7,301,335) filed by Yushi Sun and Tianhe OUYANG on Apr. 25, 2005, is titled “APPARATUS AND METHOD FOR RFEC SCANNING A SURFACE TO DETECT SUB-SURFACE CRACKS AROUND RIVETS AND THE LIKE” and is incorporated herein by reference. This patent Application describes an RFEC excitation unit and sensor apparatus and method that facilitate detection of cracks or other anomalies within or under a surface and immediately next to an expected structure (such as a rivet) that would otherwise cause a signal change and prevent detection of the cracks or other anomalies. In some embodiments, the apparatus includes actuators and control mechanisms that move the apparatus and analyze sensed RFEC signals to determine the location of the rivet, and then to rotate (mechanically or electronically) the sensed signal and/or excitation signal to maintain a constant relationship to the edge of the rivet in order that signals from the rivet edge are suppressed and signals from the cracks or anomalies are detected. In some embodiments, the excitation unit is maintained at the center of the rivet surface, and the sensor is moved around the rivet in a circle centered on the rivet.
FIG. 15 is a schematic block diagram of the excitation driver and sensing demodulator 1500 of apparatus 100. In some embodiments, eddy-current detection-and-display system 101 includes a sine-wave and cosine-wave generator 1520 that outputs sine and cosine versions of a wave having a selected frequency useful for scanning the object 99 (e.g., selected for the type of metal and its configuration and thickness). In some embodiments, the cosine(ωt) signal 1521 is buffered through adjustable amplifier 1523 having an amplification factor A, and the resulting signal A*cosine(ωt) is connected through an impedance (such as the resistance 1540 shown, or another suitable impedance such as one including resistance, capacitance, and/or inductance) to a selected coil 129 through analog switch 122 and/or mux 121, and the selected coil 129 is also connected to one input of differential amplifier 1528. In some embodiments, the cosine(ωt) signal is also buffered through adjustable amplifier 1524 having an amplification factor B, and the resulting signal B*cosine(ωt) is connected through an impedance (such as the resistance 1541 shown, or another suitable impedance such as one including resistance, capacitance, and/or inductance) to a selected reference coil 125 through analog switch 122 (and/or a mux, not shown), and the reference coil 125 is also connected to one input of differential amplifier 1528. The differential amplifier outputs a difference signal that represents the difference between the selected sense coil 129 and the selected reference coil 125 (e.g., difference signal 1593=E*cosine(ω+α)). The difference signal is mixed (i.e., multiplied) by demodulator 1525 with the cosine(ωt) signal 1521 (e.g., in some embodiments, with resulting after demodulation signal Xm 1594=cosine(ωt)*(E*cosine(ω+α))=½ E*(cosine(2ω+α)+cosine α), then goes through low-pass filter 1526 (e.g., in some embodiments, with resulting after low-pass filter signal Xp 1596=K*cosine(α), where, in some embodiments K=½ E) and H gain amplifier 1527 (e.g., in some embodiments, with resulting after horizontal gain amplifier signal X 1598=Kh*Xp (where Kh is the horizontal gain)), and represents the real component of a signal having real and imaginary components based on the phase and amplitude changes of the selected sense coil 129. The difference signal is also mixed (i.e., multiplied) by demodulator 1535 with the sine(ωt) signal 1522 (e.g., in some embodiments, with resulting after demodulation signal Ym 1595=sine(ωt)*(E*cosine(ω+α))=½ E*(sine(2ω+α)+sine(α))), then goes through low-pass filter 1536 (e.g., in some embodiments, with resulting after low-pass filter signal Yp 1597=K*sine(α), where, in some embodiments, K=½ E) and V gain amplifier 1537 (e.g., in some embodiments, with resulting after vertical gain amplifier signal Y 1599=Kv*Yp (where Kv is the vertical gain)), and represents the imaginary component of a signal having real and imaginary components based on the phase and amplitude changes of the selected sense coil 129. These two signals are then processed by rotator 1538 which has input factors cosine(beta) and sine(beta) that are used to rotate the signals by an angle (beta), which are then output as real component 1533 and imaginary component 1534.